Features of a Relational Database
Tables: The Foundation of Relational Databases
Tables
Tables are the core structure of a relational database, where data is organized into rows and columns.
Each table represents an entity, such as customers, products, or orders.Note
In a relational database, a table is often referred to as a relation, and a row is called a tuple.
Primary Keys: Ensuring Uniqueness
Primary key
A primary key is a unique identifier for each record in a table.
It ensures that no two rows have the same value in the primary key column.Example
In a customer table, the CustomerID might serve as the primary key, ensuring each customer is uniquely identifiable.
Foreign Keys: Creating Relationships
Foreign key
A foreign key is a column (or set of columns) in one table that references the primary key of another table.
This creates a link between the two tables, enabling data to be related across entities.Example
In an orders table, the CustomerID might be a foreign key referencing the CustomerID in the customer table, linking each order to a specific customer.
Composite Keys: Combining Attributes for Uniqueness
Composite key
A composite key is a primary key that consists of two or more columns.
It is used when a single column is not sufficient to uniquely identify a record.Example
In a course enrollment table, a composite key might consist of StudentID and CourseID, ensuring that each student can enroll in a course only once.
Relationships: Connecting Data Across Tables
Relationships
Relationships define how tables are connected in a relational database.
They are established through primary and foreign keys.
Types of Relationships
- Relationships might be optional or mandatory (modality).
- Mandatory Relationship:
- A relationship where a record in one table must be linked to a record in another table. The foreign key cannot be NULL.
- Example: Every student must be enrolled in a school.
Student(SchoolID) must have a valid SchoolID from the School table.
- Optional Relationship:
- A relationship where a record in one table may or may not be linked to a record in another table. The foreign key can be NULL.
- Example: A teacher might have an assigned mentor, but not all teachers do. Teacher(MentorID) can be NULL if no mentor is assigned.
- Mandatory Relationship:
- One-to-One: Each record in one table is related to exactly one record in another table.
- Example: A person and their passport.
- One-to-Many: A record in one table can be related to multiple records in another table.
- Example: A customer can have multiple orders.
- Many-to-Many: Multiple records in one table can be related to multiple records in another table.
- Example: Students enrolled in multiple courses.
Many-to-many relationships are typically implemented using a junction table that contains foreign keys from both related tables.
Benefits of Relational Databases
Data Consistency and Integrity
- Relational databases ensure data consistency by eliminating redundancy through normalization.
- Data integrity is maintained through constraints like primary and foreign keys.
When designing a relational database, always define primary and foreign keys to ensure data integrity and establish clear relationships between tables.
Reduced Data Duplication and Redundancy
- By organizing data into related tables, relational databases minimize data duplication.
- This reduces storage requirements and prevents anomalies during data updates.
- In a flat file database, a customer's address might be repeated in every order record.
- In a relational database, the address is stored once in the customer table and referenced by orders.
Efficient Data Retrieval
- Structured Query Language ( SQL) allows for complex queries to retrieve and manipulate data efficiently.
- Indexes and optimized query execution further enhance performance.
SQL is a powerful tool for querying relational databases, enabling tasks like filtering, sorting, and aggregating data with ease.
Scalability
- Relational databases can handle large volumes of data and concurrent users.
- They are designed to scale both vertically (adding more resources to a single server) and horizontally (distributing data across multiple servers).
While relational databases are scalable, they may face challenges with extremely large datasets, often referred to as "big data."
Security Features
- Relational databases offer robust security through user authentication, role-based access control, and encryption.
- This ensures that sensitive data is protected from unauthorized access.
